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        <h1 id="动机"><a href="#动机" class="headerlink" title="动机"></a>动机</h1><p>1.在软件构建过程中，我们需要为某些对象建立一种“通知依赖关系”—一个对象（目标对象）的状态发生改变，所有的依赖对象（观察者对象）都将得到通知。如果这样的依赖关系过于紧密，将使软件不能很好地抵御变化。</p>
<p>2.使用面向对象技术，可以将这种依赖关系弱化，并形成一种稳定的依赖关系。从而实现软件体系结构的松耦合。</p>
<a id="more"></a>
<hr>
<h1 id="模式定义"><a href="#模式定义" class="headerlink" title="模式定义"></a>模式定义</h1><p>定义对象间的一种一对多（变化）的依赖关系，以便当一个对象（Subject）的状态发生改变时，所有依赖于它的对象都得到通知并自动更新。</p>
<p>​                                                                                     —《设计模式》GoF</p>
<hr>
<h1 id="结构"><a href="#结构" class="headerlink" title="结构"></a>结构</h1><p><img src="/2019/06/21/designpatternobserver/observer.png" alt="观察者模式"></p>
<hr>
<h1 id="代码示例"><a href="#代码示例" class="headerlink" title="代码示例"></a>代码示例</h1><p>考虑这样一个场景，我们需要对一个大文件分割成若干个小文件。在一个界面上选择文件，并填写文件的分割数目。</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/*MainForm.cpp*/</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">MainForm</span> :</span> <span class="keyword">public</span> Form</span><br><span class="line">&#123;</span><br><span class="line">	TextBox* txtFilePath;</span><br><span class="line">	TextBox* txtFileNumber;</span><br><span class="line">	ProgressBar* progressBar;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line">	<span class="function"><span class="keyword">void</span> <span class="title">Button1_Click</span><span class="params">()</span></span>&#123;</span><br><span class="line"></span><br><span class="line">		<span class="built_in">string</span> filePath = txtFilePath-&gt;getText();</span><br><span class="line">		<span class="keyword">int</span> number = atoi(txtFileNumber-&gt;getText().c_str());</span><br><span class="line"></span><br><span class="line">		<span class="function">FileSplitter <span class="title">splitter</span><span class="params">(filePath, number, progressBar)</span></span>;</span><br><span class="line"></span><br><span class="line">		splitter.split();</span><br><span class="line"></span><br><span class="line">	&#125;</span><br><span class="line">&#125;;</span><br><span class="line"><span class="comment">/* FileSplitter.cpp */</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">FileSplitter</span></span></span><br><span class="line"><span class="class">&#123;</span></span><br><span class="line">	<span class="built_in">string</span> m_filePath;</span><br><span class="line">	<span class="keyword">int</span> m_fileNumber;</span><br><span class="line">	ProgressBar* m_progressBar;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line">	FileSplitter(<span class="keyword">const</span> <span class="built_in">string</span>&amp; filePath, <span class="keyword">int</span> fileNumber, ProgressBar* progressBar) :</span><br><span class="line">		m_filePath(filePath), </span><br><span class="line">		m_fileNumber(fileNumber),</span><br><span class="line">		m_progressBar(progressBar)&#123;</span><br><span class="line"></span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="function"><span class="keyword">void</span> <span class="title">split</span><span class="params">()</span></span>&#123;</span><br><span class="line"></span><br><span class="line">		<span class="comment">//1.读取大文件</span></span><br><span class="line"></span><br><span class="line">		<span class="comment">//2.分批次向小文件中写入</span></span><br><span class="line">		<span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; m_fileNumber; i++)&#123;</span><br><span class="line">			<span class="comment">//...</span></span><br><span class="line">			<span class="keyword">float</span> progressValue = m_fileNumber;</span><br><span class="line">			progressValue = (i + <span class="number">1</span>) / progressValue;</span><br><span class="line">			m_progressBar-&gt;setValue(progressValue);</span><br><span class="line">		&#125;</span><br><span class="line"></span><br><span class="line">	&#125;</span><br><span class="line">&#125;;</span><br></pre></td></tr></table></figure>
<p>假如我们需要添加一个进度条，用来显示分割的进度，以上的实现违背了依赖倒置原则，上层的文件分割器的依赖进度条的实现细节，此时就需要观察者模式了。</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br><span class="line">81</span><br><span class="line">82</span><br><span class="line">83</span><br><span class="line">84</span><br><span class="line">85</span><br><span class="line">86</span><br><span class="line">87</span><br><span class="line">88</span><br><span class="line">89</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/*MainForm.cpp*/</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">MainForm</span> :</span> <span class="keyword">public</span> Form, <span class="keyword">public</span> IProcess</span><br><span class="line">&#123;</span><br><span class="line">	TextBox* txtFilePath;</span><br><span class="line">	TextBox* txtFileNumber;</span><br><span class="line">	ProgressBar* progressBar;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line">	<span class="function"><span class="keyword">void</span> <span class="title">Button1_Click</span><span class="params">()</span></span>&#123;</span><br><span class="line"></span><br><span class="line">		<span class="built_in">string</span> filePath = txtFilePath-&gt;getText();</span><br><span class="line">		<span class="keyword">int</span> number = atoi(txtFileNumber-&gt;getText().c_str());</span><br><span class="line"></span><br><span class="line">		ConsoleNotifier cn;</span><br><span class="line"></span><br><span class="line">		<span class="function">FileSplitter <span class="title">splitter</span><span class="params">(filePath, number)</span></span>;</span><br><span class="line">		splitter.addProcess(<span class="keyword">this</span>);</span><br><span class="line">		splitter.addProcess(&amp;cn);</span><br><span class="line"></span><br><span class="line">		splitter.split();</span><br><span class="line"></span><br><span class="line">		splitter.removeProcess(<span class="keyword">this</span>);</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="function"><span class="keyword">virtual</span> <span class="keyword">void</span> <span class="title">DoProcess</span><span class="params">(<span class="keyword">float</span> value)</span> </span>&#123;</span><br><span class="line">		progressBar-&gt;setValue(value);</span><br><span class="line">	&#125;</span><br><span class="line">&#125;;</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">ConsoleNotifier</span> :</span> <span class="keyword">public</span> IProcess &#123;</span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line">	<span class="function"><span class="keyword">virtual</span> <span class="keyword">void</span> <span class="title">DoProcess</span><span class="params">(<span class="keyword">float</span> value)</span> </span>&#123;</span><br><span class="line">		<span class="built_in">cout</span> &lt;&lt; <span class="string">"."</span>;</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">/* FileSplitter.cpp */</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">IProcess</span> &#123;</span></span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line">	<span class="function"><span class="keyword">virtual</span> <span class="keyword">void</span> <span class="title">DoProcess</span><span class="params">(<span class="keyword">float</span> value)</span> </span>= <span class="number">0</span>;</span><br><span class="line">	<span class="keyword">virtual</span> ~IProcess() &#123;&#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">FileSplitter</span></span></span><br><span class="line"><span class="class">&#123;</span></span><br><span class="line">	<span class="built_in">string</span> m_filePath;</span><br><span class="line">	<span class="keyword">int</span> m_fileNumber;</span><br><span class="line">	<span class="built_in">vector</span>&lt;IProcess *&gt; processVector;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line">	FileSplitter(<span class="keyword">const</span> <span class="built_in">string</span>&amp; filePath, <span class="keyword">int</span> fileNumber) :</span><br><span class="line">		m_filePath(filePath), </span><br><span class="line">		m_fileNumber(fileNumber) &#123;</span><br><span class="line"></span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="function"><span class="keyword">void</span> <span class="title">split</span><span class="params">()</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">		<span class="comment">//1.读取大文件</span></span><br><span class="line"></span><br><span class="line">		<span class="comment">//2.分批次向小文件中写入</span></span><br><span class="line">		<span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; m_fileNumber; i++)&#123;</span><br><span class="line">			<span class="comment">//...</span></span><br><span class="line">			<span class="keyword">float</span> progressValue = m_fileNumber;</span><br><span class="line">			progressValue = (i + <span class="number">1</span>) / progressValue;</span><br><span class="line">			doProcess(progressValue);</span><br><span class="line">		&#125;</span><br><span class="line"></span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="function"><span class="keyword">void</span> <span class="title">addProcess</span><span class="params">(IProcess * process)</span> </span>&#123;</span><br><span class="line">		<span class="keyword">if</span> (process != <span class="literal">NULL</span>) &#123;</span><br><span class="line">			processVector.push_back(process);</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="function"><span class="keyword">void</span> <span class="title">removeProcess</span><span class="params">(IProcess *process)</span> </span>&#123;</span><br><span class="line">		<span class="keyword">if</span> (process != <span class="literal">NULL</span>) &#123;</span><br><span class="line">			processVector.erase(process);</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">protected</span>:</span><br><span class="line">	<span class="function"><span class="keyword">void</span> <span class="title">doProcess</span><span class="params">(<span class="keyword">float</span> value)</span> </span>&#123;</span><br><span class="line">		<span class="comment">// 发布通知</span></span><br><span class="line">		<span class="keyword">for</span> (<span class="keyword">auto</span> it = processVector.begin(); it != processVector.end(); ++it) &#123;</span><br><span class="line">			(*it)-&gt;DoProcess(value);</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">&#125;;</span><br></pre></td></tr></table></figure>
<hr>
<h1 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h1><p>1.使用面向对象的抽象，Observer模式使得我们可以独立地改变目标与观察者，从而使二者之间的依赖关系达到松耦合。</p>
<p>2.目标发送通知时，无需指定观察者，通知（可以携带通知信息作为参数）会自动传播。</p>
<p>3.观察者自己决定是否需要订阅通知，目标对象对此一无所知。</p>
<p>4.Observer模式时基于事件的UI框架中非常常用的设计模式，也是MVC模式的一个重要组成部分。</p>

      
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              <div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-1"><a class="nav-link" href="#动机"><span class="nav-number">1.</span> <span class="nav-text">动机</span></a></li><li class="nav-item nav-level-1"><a class="nav-link" href="#模式定义"><span class="nav-number">2.</span> <span class="nav-text">模式定义</span></a></li><li class="nav-item nav-level-1"><a class="nav-link" href="#结构"><span class="nav-number">3.</span> <span class="nav-text">结构</span></a></li><li class="nav-item nav-level-1"><a class="nav-link" href="#代码示例"><span class="nav-number">4.</span> <span class="nav-text">代码示例</span></a></li><li class="nav-item nav-level-1"><a class="nav-link" href="#总结"><span class="nav-number">5.</span> <span class="nav-text">总结</span></a></li></ol></div>
            

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